Modelling the effect of human-caused mortality on a lion sub-population using spreadsheets

[摘要] ENGLISH ABSTRACT:Free-ranging lions (Panthera leo) in the Kgalagadi Transfrontier Park (KTP) have beensubject to persecution by farmers following livestock depredation in adjacent grazing areas. Inrecent years at least one adult female was killed annually from a sub-population of fiveboundary prides that have home ranges adjoining these livestock grazing areas. While nolong-term records of human-caused mortality are available, the impact of current rates ofhuman-caused mortality is uncertain. Female-based, age-structured models were thus used toestimate the long-term viability of the KTP lion sub-population subjected to human-causedmortality under deterministic and stochastic environmental conditions. Population parametersincorporated in the models included age-class specific natural mortality, female fecundity andbirth sex ratio. In so doing sustainable threshold rates of persecution were established, so thateffective conservation measures can be taken, if required, to ensure the continued survival ofboundary prides in the park.Sensitivity analyses of natural survival rates indicated that adult female survivorship is themost important population parameter with respect to maintaining population viability,compared to younger female age-classes, fecundity or birth sex ratio. Hence adults were alsothe most sensitive age-class with respect to human-caused mortality, as adult survivalrepeatedly acts upon individuals with the highest reproductive value. In the deterministicmodel, with the most optimistic survival parameter values, fecundity and birth sex ratio(female-biased) estimates, the sub-population is only able to sustain an annual persecution ofthree adult females, before the sub-population exhibits a sustained decline. In the worst-casescenario, where fecundity and sex ratio estimates are at their lower extremes, the maximumsustainable age-class specific persecution rate is zero, for all age-classes. Whilst thesehypothetical scenarios are unrealistic, they do highlight the extreme thresholds of potentiallysustainable persecution rates. Under the most optimistic scenario using the stochastic model,the highest achievable survival probability of the sub-population, when subjected to apersecution rate of one adult annually, was 78%. Although increased fecundity and birth sexratio biased towards females may increase the survival probability, these parameters aregenerally at their mean values in the long-term, and may thus not necessarily prevent asustained population decline. The models therefore suggest that the current persecution rate ofone adult annually (or 4% of the adult sub-population), appears unsustainable in the longterm.To ensure the survival of existing boundary prides and to maintain a viable subpopulation,adult lionesses should, as far as possible, be afforded protection from persecution.

[发布机构] Stellenbosch University